Recurring error

Hi dear support,

I get error notifications every time I log in as an administrator.

An error of type E_ERROR has been caused in line 44 of the file /*/*/*/* /wp-content/plugins/ninjafirewall/lib/email_sodium.php. Error message : Uncaught Error: Undefined constant “AUTH_KEY” in /wp-content/plugins/ninjafirewall/lib/email_sodium.php:44

On the other hand, the plugin is working without any problem.

I don’t know how to interpret this error, if it is one.

Thanks for any help on this matter.